This invention relates to a process for crosslinking polyalkylene polymers, particularly polyethylene and polypropylene polymers. The process employs steps of irradiation and, preferably, annealing. The invention further relates to crosslinked polyalkylene polymers produced by the process.
There have been earlier proposals to crosslink polyalkylene polymers, by processes which employ UV radiation. However, there is an inherent difficulty, as it is deemed necessary to incorporate the photoinitiator and the crosslinking agent in the melt blend prior to the ionizing step. In the Journal of Applied Polymer Science, 56, pp 935-946, (1995), Zamotaev et.al. have used Xanthone (XN) to photoinduce crosslinking with triallyl cyanurate (TAC) in LDPE, PP and LDPE/PP blends, pressed into sheets 0.4 mm thick. These authors use high concentrations of the additives (XN.apprxeq.0.3% and TAC.apprxeq.2.0%) pre-mixed in the polymer blends before melt compounding. In addition, the irradiations were carried out at 170.degree. C. in air. Japanese patent application number 41865651, Japanese patent application number 3296540, Japanese patent application number 3296539 and Japanese patent application number 4089844 disclose the use of benzophenone and/or derivatives of benzophenone and triallyl isocyanurate as a photosensitiser and a crosslinking agent respectively. The photosensitiser and crosslinking agent are incorporated with the crystalline polyolefin as part of the moulded part and the moulded part is irradiated using UV light of different frequencies.
Both techniques are limited by the requirement to incorporate the photoinitiator and the crosslinking agent in the polymer matrix prior to irradiation.
JP 59053540 describes the technique of coating a polyethylene film with a photosetting mixture which includes a photopolymerisation initiator such as benzophenone and a crosslinkable monomer. After coating, UV irradiation takes place to harden the surface of the film.
DE 2165438 describes how a polyethylene foil is brushed with a UV sensitiser solution and crosslinked by irradiation with UV light. The sensitiser solution includes benzophenone in an organic solvent. The polyethylene foil is said to be crosslinked by the process.
Both of these methods suffer from the drawback of a lack of penetration into the polymer matrix and are only directed to surface treatment of the polymer.